Method and device for descaling a scaled metal strip

ABSTRACT

The invention relates to a method for descaling a scaled metal strip, in particular a hot-rolled metal strip with a thickness of more than 1 mm, for example 1.2 mm to 16 mm. The strip is guided around at least two straightening rollers which are arranged one behind the other in the strip running direction and undergoes an alternating bending in the process. At least one of the straightening rollers can be positioned transversely to the strip running direction. The method is characterized in that the strip is unstressed or is under a low tensile stress of less than 10% of the yield strength of the strip on the inlet side upstream of the straightening rollers, and the diameter of the straightening rollers, the spacing between same in the strip running direction, and the positioning of the rollers transverse to the strip running direction are selected while taking into consideration the strip properties such that expansions and compressions of at least 2%, preferably at least 3%, are produced in the course of a first plastic bending process and a second plastic bending process on the strip upper face and on the strip lower face for the purpose of descaling.

The invention relates to a method of and an apparatus for descaling metal strip, in particular a scaly hot-rolled metal strip having a thickness of more than 1 mm, for example 1.2 mm to 16 mm, where the strip is guided around at least two straightening rolls one downstream of the other in a travel direction of the strip so as to undergo alternate bending, at least one of the straightening rolls being positionable transversely to the travel direction of the strip. Within the context of the invention, metal strip means in particular metal strip made from steel or stainless steel. These may be strips that are wound onto coils. In the context of the invention, however, metal strip also relates to metal sheets or similar strip sections.

In particular hot-rolled metal strips have to be descaled, for example by pickling or by other measures. To improve this descaling process it is known to break up the scale on the strip surface by mechanical means, for example in order to ensure an improved acid attack during pickling. In practice, descaling is as a rule carried out by stretch bending. Apart from the efficient mechanical pre-descaling with the resulting higher process rates and lower acid consumption, stretch bending is also useful for removing any unevenness in the strip caused by plastic stretching.

Such methods of descaling hot-rolled strips by stretch bending are known for example from DE 23 50 503 [U.S. Pat. No. 3,924,428] and DE 29 42 270. In order to generate the strip tension required for stretch bending at the upstream and downstream ends, such stretch-bending systems are equipped with tensioning roll sets at the upstream and downstream ends. Due to the deformation losses that disproportionately increase with the strip thickness, the strip tension at the downstream end is as a rule markedly higher than the strip tension at the upstream end. In practice, stretching degrees of more than 1% are usually required in order to achieve an efficient descaling and at the same time an improved stretching result. From a system engineering point of view, this requires relatively high capital investment, namely in particular due to the required tensioning roll sets. Moreover, descaling using stretch-bending systems is limited to strip treatment lines with an endless strip, so that the strip leading and trailing ends of the coils have to be joined together.

Alternatively, DD 294 886 proposes an apparatus for is mechanically pre-descaling hot strip that is fed from an upstream supply coil to a pickling bath, which apparatus is made up of a plurality of bending mills each equipped with a concave and a convex bending roll, which bending roll pairs change from one bending rolling mill to another bending rolling mill according to their roll profile, so that the continuous profile between the bending rolls is alternately curved upward and curved downward. It should be possible to use such an apparatus also in push-type pickling systems, where transport may be carried out for example by driven roll sets. As a result of the alternating bending roll profiles, the strip is alternately deformed as it travels downstream, with the bending occurring as a result of the convex-concave profiles substantially about an axis that extends in the travel direction of the strip. Here, the bending geometry is dictated by the roll geometry, so that the system can only insufficiently be adapted to different circumstances.

DE 10 2009 035 161 describes a method and an apparatus for descaling metal strips in which a cryogen (for example liquid nitrogen) is applied in order to release and remove contaminant and/or scale particles. Moreover, water is applied to the strip surface prior to the application of the cryogen. During the application of the water or after the application of the water and prior to the application of the cryogen, the strip may be guided over at least one roll at least on one side, so that the strip is subjected to bending deformation.

Further, a method of descaling hot-rolled steel sheets is known in which the metal sheet is mechanically deformed for the is purpose of breaking up the oxide layer, and the broken-up oxide layer is carried off by a yieldable grinding pad. In the course of this, the metal sheet is bent to a radius between 50 mm and 300 mm. The metal sheet is stretched to at least 0.2% of permanent elongation (see DE 198 17 525).

Apart from that, an apparatus for straightening metal strip using guide rolls that are parallel to each other and a straightening roll disposed between the two guide rolls is known in which the strip is wrapped in a form-fit manner around the straightening roll between two lines of contact, along which the guide rolls are in indirect contact with the guide rolls. The straightening is carried out by altering the bending shape, so that the strip tension does not need to perform any straightening, but is only used to advance the strip. The required tensile force remains well below what is required for stretching and bending the strip during stretch bending. Here, the guide rolls together with the straightening roll form a straightening path and in doing so occupy the space from the straightening roll, which is determined by the strip thickness, so that the strip is clamped along the two contact lines in a gap as it were between the straightening roll on the one hand and respectively one guide roll on the other hand (see EP 0 865 839 [U.S. Pat. No. 5,953,946] or DE 197 08 488 [U.S. Pat. No. 5,815,034]).

Such a development of this straightening by “form-fit bending” is described for example in DE 199 17 561. Such developments for form-fit bending have no bearing on the technology of descaling.

Finally, a method of descaling is also known from US 2010/0146757 [U.S. Pat. No. 8,707,529].

The object of the invention is to provide a method that allows problem-free and economical descaling at reduced system (investment expenses) for a perfect and effective descaler.

To attain this object, the invention teaches, in a method of the above-mentioned type for descaling metal strip, in particular of a hot-rolled metal strip, that the strip is without tension at the upstream end upstream of the straightening rolls or is subject to a low tensile stress of less than 10% of the yield strength of the strip and/or of less than 25 MPa and that the diameter of the guide rolls and the spacing thereof in the travel direction of the strip as well as their positioning transversely to the travel direction of the strip are selected such, taking into account the strip properties (and the tensile stress), that elongations and compressions of at least 2%, preferably of at least 3% are generated on the upper face and the lower face of the strip.

The invention is based here on the discovery that problem-free descaling becomes possible even without any degree of stretching and consequently without any plastic elongation of the strip, if as a result of a tension-free alternate bending, the elongations and compressions on the upper face and the lower face of the strip are sufficiently great. Any overall plastic elongation of the strip, which occurs during stretch bending, is dispensed with. However, the pickling attack of the acid or the effect of any mechanical descaling methods such as brushing or spraying can be markedly increased even without any degree of stretching. According to the invention, the strip is plastically bent at least once in one direction and once in the opposite direction, so that the upper face and the lower face of the strip are both plastically elongated and plastically compressed. In order to achieve approximately the same descaling effect on both sides, approximately equal bending radii are preferably set in each case for the two bends. As a result of the tight placement of the rolls, a bending elongation or an edge-fiber elongation/compression of more than 2%, preferably more than 3% is required even in the case of a tension-free passage or if the strip is subjected to no or only a very low tensile stress. The invention is based here on the realization that these bending elongations can be readily generated if the rolls are very close to each other. However, any “form-fit bending”, which is described for example in EP 0 865 839, is expressly dispensed with. Rather, the spacing between the roll surfaces of two directly adjacent rolls is always greater than the strip thickness, so that no form-fit bending takes place. This has the advantage that the scale broken up and partially flaking or dropping off can be disposed of, for example, in a dry method by suction or in a wet method by flushing away, for example using water. This is possible despite the tight arrangement of the straightening rolls, so that any accumulation or caking of scale on the straightening rolls and any associated surface damage on the strip is avoided. The reason is that the two-sided contact of the strip with the straightening roll and any idler roll, which occurs during form-fit bending and in the case of a scaled strip would lead to surface damages, is avoided.

Preferably, an idler roll (or a further straightening roll) is provided upstream of the upstream straightening roll. An idler roll (or a further straightening roll) is preferably provided downstream of the downstream straightening roll. Alternatively, an idler roll may be provided both upstream or downstream of each straightening roll, however these (additional) idler rolls can also be replaced with (additional) straightening rolls. One or more idler rolls (or additional straightening rolls) may also be positionable transversely to the travel direction of the strip. Thus, the straightening roll may either be moved relative to one or more (fixed) idler rolls or the idler roll may be moved relative to a (fixed) straightening roll. Alternatively, both the straightening rolls and the idler rolls may be positionable. In any case, a very compact arrangement of the straightening rolls and idler rolls is carried out, so that the desired elongations or compressions on the upper face and/or the lower face of the strip are generated. As a result of the tight arrangement of the straightening rolls and the idler rolls, an edge-fiber elongation/compression of more than 2%, preferably more than 3% is enforced even in the case of a tension-free passage or if the strip is subjected to a low tensile stress. Edge-fiber elongation/compression means elongation/compression on the upper face and the lower face of the strip.

The spacing of the straightening rolls and, if present, the idler rolls and the positioning of the straightening rolls and, if present, the idler rolls is here selected such that in the tension-free condition, the strip can assume on the straightening rolls a maximum circle diameter on the straightening rolls that corresponds to a maximum of three times the straightening roll diameter, preferably twice the straightening roll diameter. Consequently, the straightening rolls and idler rolls are located in such close proximity to each other and are positioned at such a transverse overlap depth that even in the case of a tension-free strip, a maximum circle diameter of the strip may be formed that cannot exceed a maximum value of three times the straightening roll diameter, preferably even only twice the straightening roll diameter. The bending line of the strip, which occurs as a result of the bending moments and, if present, the (low) tensile stress, will then result in a bending radius that corresponds, for thinner strips, to the straightening roll radius. In light of this it is useful if the straightening roll diameter is selected to be so small that even in the case of the smallest strip thickness, a(n) (surface) elongation of more than 2%, preferably more than 3% will be achieved. To this end, it is useful if the straightening rolls have a diameter that has a maximum of fifty times the smallest strip thickness that is to be processed in the system. For thicker strips, the bending radius will be adjusted by positioning the straightening rolls and, if present, the idler rolls in such a way that the maximum edge-fiber elongation or surface elongation of more than 3% is achieved.

Under consideration of the fact that bending losses occur on the straightening rolls, which as a rule will increase with the strip thickness, the invention proposes creating a strip tension at the downstream end and consequently downstream of the straightening rolls that exceeds the strip tension at the upstream end at least by the degree of the deformation losses. Consequently, the strip will be under a tensile stress that is sufficient to cover, in cooperation with any idler roll drives, all of the deformation losses that may occur in the apparatus. As a rule, the strip tension at the downstream end is generated by the wind-up tension of a finish coil. In the case of very large strip thicknesses, the deformation losses may become so great that the strip tension required at the downstream end exceeds the available tension (for example the wind-up tension). In this case, the positioning of the straightening roll will then be limited or reduced.

In any case, the apparatus according to the invention can also be used in lines without an endless strip, for example in push-type pickling lines. Such lines are predominantly used for great strip thicknesses of for example up to 16 mm. In the case of such an push-type pickling line, as a rule the strip leading end will be pushed from the supply coil via a drive unit at the upstream end through the pickling section to the finish coil. It is useful here, for example in an push-type pickling line, to drive the strip leading end through the descaling apparatus, and in this case the positionable straightening rolls (and, if present, idler rolls) are initially spread, so that the strip leading end can readily be fed through between the rolls. It is possible to advance the strip leading end up to the finish coil where the strip tension required at the downstream end is generated. Subsequently, the straightening rolls (and, if present, the idler rolls) are moved in. The strip length from the finish coil back to the descaling apparatus will in this case not be pre-descaled. Therefore, at least one further drive unit (at the downstream end) may alternatively be provided at the downstream end immediately downstream of the descaling apparatus to generate the required strip tension. As soon as the strip leading end has passed the drive unit (at the downstream end) the latter is closed and strip tension is built up. Consequently, as a result of the additional drive unit at the downstream end, the amount of strip length that is not pre-descaled can be minimized. The same applies to the strip trailing end that runs through the descaling apparatus. Starting from a certain number of windings remaining on the supply coil, the unwinding tension and thus the tensile stress acting at the upstream end in the strip may be lost. Thus it may occur that the bending radius on the straightening rolls increases and the descaling effect decreases. To prevent this, the invention proposes as an option to provide a drive unit at the upstream end that generates the strip tension at the upstream end for the passage of the strip, so that also the strip trailing end, with the exception of the strip length from the driving unit at the upstream end up to the straightening rolls, is optionally pre-descaled.

In this context it falls within the scope of the invention to drive the straightening rolls and/or idler rolls for example in order to compensate for part of the deformation losses.

According to a further suggestion of the invention it is provided for the last straightening roll (or the last idler roll) to have a positionable bending roll disposed downstream thereof that can correct any residual curvature. In this context, the invention proceeds from the realization that, as a result of the high plastic bending about the straightening rolls in the strip, a plastic residual curvature is generated. This will be removed or minimized by the positioning of at least one positionable bending roll. Consequently, the strip will be counter-bent after the plastic bending about the straightening rolls by at least one bending roll, so that no or only a small residual plastic curvature remains in the strip. Such a bending roll will preferably have a diameter that is dimensioned such that even the smallest strip thickness can still be bent beyond the elastic limit. Optionally, the bending roll may be driven.

It falls within the scope of the invention that the scale broken up on the straightening rolls is aspirated or is flushed or blown off by a (liquid or gaseous) medium. In this way, any accumulations or deposit build-ups in the area of the straightening rolls will be avoided, so that in particular surface damage to the strip is avoided.

According to the invention, the straightening rolls have a relatively small diameter. Preferably, the straightening rolls have a diameter that is less than fifty times the smallest strip thickness to be processed. It may thus be useful to use straightening rolls having a diameter of less than 100 mm. At best, the straightening roll diameter would be selected to be so small that, even in the case of the smallest strip thickness, a sufficient edge-fiber elongation will be achieved. It is useful here to support the straightening rolls against sagging by backup rolls, with each straightening roll preferably having two backup rolls associated therewith. The ratio between the backup roll diameter and the straightening roll diameter is preferably 1.5, particularly preferably 1.25. Consequently, it falls within the scope of the invention to use backup rolls with a relatively low diameter, so that the desired tight geometry can be realized. In contrast to multiroll straightening, where two straightening rolls placed one downstream of the other can be supported by a common “interposed” backup roll, the invention preferably proposes to associate in each case at least two separate respective backup rolls with the straightening rolls or with each straightening roll. Consequently, the individual straightening rolls are in each case individually supported, so that each backup roll is associated with just one single straightening roll.

The idler rolls preferably have a diameter of 150 mm to 600 mm, particularly preferably of 200 mm to 300 mm. In the case of such diameters, idler rolls can be created that on the one hand can accommodate sufficiently great forces and on the other hand allow the tight geometry of the arrangement that is essential to the invention.

The object of the invention is not only the method described, but also an apparatus for carrying out this method. Such an apparatus includes at least one upstream straightening roll and one downstream straightening roll. Preferably, at least one idler roll is provided upstream of the upstream straightening roll and at least one idler roll is provided downstream of the downstream straightening roll. Optionally, one idler roll can be provided upstream of each straightening roll and one idler roll can be provided downstream thereof. The apparatus preferably further includes a supply coil at the upstream end and a finish coil at the downstream end, with the finish coil as a rule not being provided directly downstream of the descaling apparatus, but downstream of the descaling or pickling section. Consequently, a descaling apparatus is provided downstream of the descaling apparatus, and this descaling apparatus may for example be a pickling apparatus. Alternatively, however, also other descaling apparatus may be used, for example blasting apparatus. According to a further proposition there is downstream of the last straightening roll (or the idler roll) a drive unit at the downstream end. Alternatively or in addition, a drive unit at the upstream end may be provided upstream of the upstream straightening roll (or idler roll).

In order to ensure automatic operation of the system and a setting of approximately the same bending radii on the two straightening rolls, it is advantageous if, for given strip data, the optimal roll positioning is precalculated using a mathematical model that takes into account as parameters at least the strip thickness, the strip width, the yield strength, the elasticity module, the roll diameter as well as the roll geometry, the tensile stress on the strip at the upstream end and the occurring deformation bending losses.

It falls within the scope of the invention that for the desired strip thickness range an upstream straightening roll and a downstream straightening roll, i.e. a straightening roll pair is provided in which the straightening rolls may be directly adjacent one another in the travel direction of the strip, or wherein each straightening roll has two respective idler rolls. In order to extend the range of use of such a system, it is also possible to provide alternatively two straightening roll pairs or even more than two straightening roll pairs, the straightening rolls of the upstream straightening roll pair having a different diameter than the straightening rolls of the downstream straightening roll pair. It therefore becomes possible to use each of these straightening roll pairs for a certain strip thickness range, so that descaling can be optimized. Thus, it falls within the scope of the invention to provide an upstream straightening roll pair that includes straightening rolls having a straightening roll diameter that is smaller than the straightening roll diameter of the straightening rolls of a downstream straightening roll pair. The upstream straightening roll pair will then be used for descaling thinner strips, whereas the downstream straightening roll pair with a greater straightening roll diameter will be used for descaling thicker strips. In this case it is also useful if the respective backup rolls have a smaller or a larger diameter. In any case, the bending effect can be enhanced with smaller straightening roll diameters on thinner strips. However, for the bending forces occurring on thicker strips, the additional loads on the backup rolls will be exceeded. Therefore, it is advantageous in the case of thicker strips to use a downstream straightening roll pair with larger diameters.

It may also be prudent to use two straightening roll pairs, if the bending radii occurring in the case of a tension-free strip at the upstream end or in the case of very low tensile stresses at the upstream end are still too great to achieve an effective descaling. As a result of the deformation losses that occur during bending about the upstream straightening roll pair, the tensile stress on the strip will be increased after the upstream straightening roll pair and thus the tensile stress on the strip at the upstream end for the downstream straightening roll pair. As a result, smaller bending radii will be achieved in the case of strips where the bending radius is greater than the straightening roll diameter, i.e. the surface elongation and compression and thus the descaling effect will increase. If necessary, it is also possible to use a plurality of straightening roll pairs for increasing the strip tension. As a result of increasing the strip tension, also a (low) degree of stretching will be generated in the strip, so that the flatness of the strip is improved.

Particularly preferably, the descaling apparatus according to the invention can also be used in lines without an endless strip, for example in push-type pickling lines. Alternatively, the descaling apparatus according to the invention can also be used in lines with an endless strip as a low-cost variant compared to a stretch-bending system.

The invention will be explained below in more detail with reference to a drawing that illustrates an embodiment. Therein:

FIG. 1 is a highly simplified view of an apparatus for descaling hot-rolled strips using a descaling apparatus,

FIG. 2 is an enlarged view of the descaling apparatus,

FIG. 3 shows a modified embodiment of the apparatus of FIG. 2, and

FIG. 4 shows a further embodiment of the apparatus of FIG. 2.

FIG. 1 shows an apparatus for descaling a hot-rolled scaly strips 1 that unwound from a supply coil and is wound onto a finish coil 5. Predescaling is carried out in a (mechanical) descaling apparatus 3, in which the scale on the strip surface is broken up in order to ensure an improved acid attack in the downstream descaling or pickling section 4. The subject matter of the present invention is in particular the descaling apparatus 3 that is indicated only schematically in FIG. 1. In FIGS. 2 to 4, various embodiments of this descaling apparatus 3 are shown. The descaling apparatus 3 has at least one straightening roll pair P with two straightening rolls 6 and 7 arranged one downstream of the other in the travel direction of the strip, the metal strip 1 being guided around these straightening rolls 6 and 7 and subjected to alternate bending. In the embodiment shown in FIG. 2, there is an idler roll 8 upstream of the upstream straightening roll 6 and an idler roll 9 downstream of the downstream straightening roll 7. In the embodiment shown, the (upper) straightening roll 6 and the (upper) idler roll 9 can be moved transversely to the travel direction of the strip while the (lower) straightening roll 7 and the (lower) idler roll 8 may be stationary. The straightening rolls 6 and 7 are each braced by backup rolls 12. Each straightening roll has associated therewith two respective backup rolls and each backup roll is associated with a single straightening roll. The fact that the rolls can be positioned transversely to the travel direction of the strip is indicated by double arrows.

According to the invention, descaling is carried out by bend-straightening, that is by alternate bending around the straightening rolls 6 and 7, strip 1 being untensioned on the intake side upstream of the straightening rolls 6 and 7 and the idler rolls 8 and 9, or is only subjected to a low tensile stress of less than 10% of the yield strength of the strip and preferably less than 25 MPa. The figures show that the diameter of the straightening rolls and the spacing thereof in the travel direction of the strip, as well as the spacing from the respective straightening rolls as well as the positioning of the straightening rolls and the idler rolls is selected such that in the course of a first plastic bending about the straightening roll 6 and a second plastic bending about the straightening roll 7, elongations and compressions of at least 2%, preferably at least 3% are generated on the top and the bottom face of the strip. According to the invention, descaling is carried out by tension-free alternate bending, which means by contrast to stretch bending, any overall plastic elongation (degree of stretching) is dispensed with. However, an efficient breaking up of the scale, so that the pickling attack of the acid or the effect of the downstream mechanical descaling techniques such as brushing or spraying without any degree of stretching is markedly enhanced. According to the invention, this is achieved by the tight arrangement of the straightening rolls 6 and 7 relative to each other or the straightening rolls 6 and 7 relative to the idler rolls 8 and 9, is under consideration of the roll diameters. As a result of the tight arrangement of the idler rolls 8 and 9 and the straightening rolls 6 and 7, a bending elongation of more than 2%, preferably more than 3% is required in the case of a tension-free passage or with the strip under low tensile stress. The spacings of the idler rolls 8 and 9 and of the straightening rolls 6 and 7 and the positioning or plunging depth thereof will here be adjusted in such a way that the strip 1 can adopt a maximum circle diameter K on the straightening rolls 6 and 7, which corresponds to a maximum of three times the straightening roll diameter, preferably to a maximum of twice the straightening roll diameter. This is indicated in the figures by the (dotted) line K. Even if the strip 1 is guided through the arrangement shown without any tension, the strip can still maximally adopt the indicated circle diameter K, due to the tight arrangement of the straightening rolls 6 and 7 and the idler rolls 8 and 9 and the adjusted plunge depths.

In the illustrated embodiment, a bending roll 11 is provided downstream of the straightening roll 7 at the downstream end or on the idler roll 9 thereof for correcting any residual curvature. A further idler roll 10 is provided between the idler roll 9 and the bending roll 11 on the opposite side of the strip.

Apart from that it can be seen that there is a drive unit 13 at the upstream end and a drive unit 14 at the downstream end. In an push-type pickling line it is possible to drive the leading end of a coil through the descaling apparatus 3, whereby initially the positionable straightening rolls and idler rolls are spread. In the embodiment according to FIG. 2, these are the straightening roll 6 and the idler roll 9 as well as, if present, the bending roll 11 that are initially spread. The strip leading end can in principle be advanced up to the finish coil 5 that generates the required strip tension at the downstream end. It is only then that the straightening rolls 6 and the idler rolls 9 are positioned. In this case, the strip length up to the finish coil 5 back to the descaling apparatus 3 would not be pre-descaled. The drive unit 14 provided at the downstream end directly downstream of the descaling apparatus minimizes the strip length that is not pre-descaled. The reason is that the required strip tension can be generated by this drive unit 14. As soon as the strip leading end has passed the drive unit 14, it closes and the strip tension builds up. The same applies to the trailing end of a coil that runs through the descaling apparatus 3. As soon as, starting from a certain number of remaining turns on the supply coil, the unwinding tension and thus the tensile stress acting in the strip at the upstream end is lost, there is in principle the possibility that the descaling effect decreases. In order to prevent this, the drive unit 13 is installed at the upstream end, which generates strip tension at the upstream end for the strip 1, so that also the strip trailing end, with the exception of the strip length from the drive unit 13 at the upstream end up to the straightening rolls, is optimally pre-descaled.

FIG. 3 shows a modified embodiment of the invention, where an upstream straightening roll 6 and a downstream straightening roll 7 for alternate bending are also provided. While according to FIG. 2, only one idler roll 8 is provided is upstream of the upstream straightening roll 6 and only one idler roll 9 is provided downstream of the downstream straightening roll 7, FIG. 3 shows an embodiment where two idler rolls 8, 8′ and 9, 9′ are associated with each straightening roll 6 and 7. The idler rolls 8, 8′ are associated with the straightening roll 6 and are provided on the opposite side of the strip, and the idler rolls 9, 9′ are associated with the straightening roll 7 and are also provided on the opposite side of the strip. In this case it is useful if the two straightening rolls 6 and 7 are positionable, whereas the idler rolls 8, 8′ and 9, 9′ may be stationary. Positionability is again indicated by double arrows.

FIG. 4 shows a further option of the invention. In this embodiment according to FIG. 4, two straightening roll pairs P, P′ are provided each with two straightening rolls 6 and 7, so that the embodiment according to FIG. 4 ultimately corresponds to a “doubling” of the embodiment according to FIG. 2. The upstream straightening roll pair P has straightening rolls 6 and 7 with a small diameter of for example 50 mm to 60 mm, whereas the downstream straightening roll pair P′ has straightening rolls 6 and 7 with a larger diameter of for example 70 mm to 80 mm. The diameter of the idler rolls may be for example 250 mm. Consequently, the descaling apparatus according to FIG. 4 may cover a particularly large range of strip thicknesses, because the straightening roll pairs P and P′ can be employed for different strip thicknesses. For thinner strips, for example the upstream straightening roll pair P may be used and for thicker strips the straightening roll pair P′ with larger straightening rolls and is backup rolls may be used. The reason is that when smaller straightening roll diameters are used, the bending effect may be enhanced for thinner strips. For bending forces that occur with thicker strips, however, the additional loads on the backup rolls are exceeded, so that it is advisable with thicker strips to use a downstream straightening roll pair with larger diameters. Alternatively, however, it may also be useful to use both straightening roll pairs P and P′, namely when the bending radii occurring in the case of a tension-free strip at the upstream end or in cases of very low tensile stresses at the upstream end are still too great for an effective descaling. As a result of the deformation losses occurring when bending about the upstream straightening roll pair P, the tension in the strip increases after the upstream straightening roll pair P and thus the strip tension at the upstream end for the downstream straightening roll pair P′. As a result, smaller bending radii occur in the case of strips where the bending radius is greater than the straightening roll diameter, which means the edge-fiber elongation and compression and thus also the descaling effect increase. 

1. A method of descaling metal strip, in particular a hot-rolled metal strip having a thickness of more than 1 mm, the method comprising the steps of: guiding the strip upstream and downstream straightening rolls arranged one downstream of the other in the travel direction of the strip such that the strip undergoes an alternate bending, at least one of the straightening rolls being movable transversely to the travel direction of the strip; maintaining the strip tension-free upstream of the straightening rolls at the upstream end or at a low tensile stress of less than 10% of the yield strength of the strip; and using straightening rolls of such a diameter and spacing in the travel direction of the strip as well and positioning the straightening rolls transversely to the travel direction of the strip under consideration of the strip properties such that in the course of a first plastic bending at the upstream straightening roll and of a second plastic bending on the downstream straightening roll, elongations and compressions of at least 2% are generated for descaling on the upper face and the lower face of the strip.
 2. The method as claimed in claim 1, further comprising the step of: providing an idler roll upstream of the upstream straightening roll; providing a transversely positionable idler roll downstream of the downstream straightening roll; transversely positioning at least one of the idler rolls and spacing the straightening rolls and idler rolls in the travel direction such that in the course of the plastic bending, elongations and compressions of at least 2% are generated on the upper face and the lower face of the strip.
 3. The method as claimed in claim 2, further comprising: selecting the spacing of the straightening rolls and of the idler rolls and the positioning of the straightening rolls and the idler rolls such that in its tension-free condition on the straightening rolls, the strip can adopt a maximum circle diameter that corresponds to a maximum of three times the straightening roll diameter.
 4. The method as claimed claim 1, wherein the bending radii are the same or substantially the same in the course of the first plastic bending and of the second plastic bending.
 5. The method as claimed in claim 1, further comprising the step of: generating a strip tension at the downstream end that exceeds the strip tension at the upstream end at least by the extent of deformation losses.
 6. The method as claimed in claim 2, further comprising the step of: setting the spacing between the roll surfaces of two immediately adjacent straightening rolls or idler rolls to be always greater than the strip thickness.
 7. The method as claimed in claim 1, further comprising the step of: providing a positionable bending roll downstream of the last straightening roll so as to correct any residual curvature.
 8. The method as claimed in claim 1, wherein the straightening rolls have a diameter that is less than fifty times the smallest band thickness.
 9. The method as claimed in claim 1, further comprising the step of: subjecting the strip to a tensile stress of less than 25 MPa upstream of the straightening rolls.
 10. The method as claimed in claim 1, further comprising the step of: aspirating or flushing away scale broken up on the straightening rolls.
 11. The method as claimed in claim 1, further comprising the step of: bracing each of the straightening rolls the strip is guided around by at least two respective backup rolls.
 12. An apparatus for descaling a hot-rolled metal strip having a thickness of more than 1 mm, the apparatus comprising: at least one upstream straightening roll; a downstream straightening roll downstream thereof; an idler roll upstream of the downstream straightening roll; an idler roll downstream of the downstream straightening roll, drive means for maintaining the strip tension-free at the upstream end upstream of the straightening rolls or at a low tensile stress of less than 10% of the yield strength of the strip, the diameter of the straightening rolls and the spacing thereof in the travel direction of the strip as well as the positioning thereof transversely to the travel direction of the strip being such that, under consideration of the strip properties, in the course of a first plastic bending and of a second plastic bending on the upper face and the lower face of the strip, elongations and compressions of at least 2% are generated.
 13. The apparatus as claimed in claim 12, wherein the idler rolls have a larger diameter than the straightening rolls.
 14. The apparatus as claimed in claim 12, further comprising: respective upstream and downstream pairs of backup rolls supporting the straightening rolls.
 15. The apparatus as claimed in claim 14, wherein a ratio between the backup roll diameter and the straightening roll diameter amounts to a maximum of 1.5.
 16. The apparatus as claimed in claim 12, wherein the idler rolls have a diameter of 150 mm to 600 mm.
 17. The apparatus as claimed in claim 12, further comprising: a positionable bending roll downstream of the last straightening roll and capable of effecting a correction of any residual curvature.
 18. The apparatus as claimed in claim 12, further comprising; at least one supply coil and one finish coil and respective drive units upstream and downstream of the straightening rolls at the upstream end and at the downstream end.
 19. The apparatus as claimed in claim 12, further comprising: respective upstream and downstream straightening roll pairs the straightening rolls of the upstream straightening roll pair having a different diameter from that of the straightening rolls of the downstream straightening roll pair, at least one of the straightening roll pairs being positionable. 